This invention relates to phosphors and more particularly to electroluminescent phosphors. Still more particularly, it relates to a process for improving the half-life of such phosphors and particularly, such phosphors that emit white light.
Phosphors are a known class of materials that emit electromagnetic radiation in response to stimulation by a form of energy. Generally, the emission of the phosphors is in the visible region of the spectrum. These phosphors are delineated by the form of energy to which they are susceptible, thus: cathode ray tube phosphors are stimulated to emission by impingement of electrons; photoluminescent phosphors are stimulated to emission by actinic radiation; x-ray phosphors are stimulated to emission by the impingement of x-rays; and electroluminescent (hereinafter, EL) phosphors are stimulated to emission by placement in an electric field. It is the latter type of material to which this invention is directed, particularly such phosphors based on ZnS and activated with one or more other materials such as Mn, Cu and/or Cl.
EL phosphors have many commercial uses such as for LCD backlights, copy machines, automotive dashboard displays, nightlights, control switch illumination, emergency lighting, watch dials, etc. These phosphors individually have emission colors ranging from orange-yellow through green to greenish-blue and blue.
It is known to combine at least three such phosphors to make a material that will emit white light; however, the half-life of the white-emitting phosphor blends is not great, only about 700 hours. Since half-life is important in commercial applications of EL phosphors, it would be advantageous to provide a white-emitting phosphor blend having a longer half-life.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the half-life of white emitting electroluminescent phosphors.
These objects are accomplished, in one aspect of the invention, by providing a phosphor blend comprising about 75 wt. % of a first electroluminescent phosphor having an x color coordinate of about 0.530 and a y color coordinate of about 0.455; about 10 wt. % of a second electroluminescent phosphor having an x color coordinate of about 0.185 and a y color coordinate of about 0.440, said second electroluminescent phosphor containing about 600 ppm copper; and about 15 wt. % of a third electroluminescent phosphor having an x color coordinate of about 0.160 and a y color coordinate of about 0.175, said third electroluminescent phosphor containing about 300 ppm copper.
This phosphor blend, when incorporated into an electroluminescent lamp, and energized by a suitable alternating current electric field, emits white light having an x color coordinate of from about 0.325 to about 0.335 and a y color coordinate of from about 0.355 to about 0.360. Further, the half-life of the phosphor is greater than about 750 hours, and more preferably greater than about 1000 hours.